This invention relates generally to gas turbine engines, and more specifically to gas turbine engine assemblies and methods of assembling the same.
At least some known gas turbine engines include a fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, a high-pressure turbine and a low-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a shaft to define a high-pressure rotor assembly. Air entering the core engine is mixed with fuel and ignited to form a high energy gas stream. The high energy gas stream flows through the high-pressure turbine to rotatably drive the high-pressure turbine such that the shaft, in turn, rotatably drives the compressor.
The gas stream expands as it flows through the low-pressure turbine positioned aft of the high-pressure turbine. The low-pressure turbine includes a rotor assembly having a fan coupled to a drive shaft. The low-pressure turbine rotatably drives the fan through the drive shaft.
Modern commercial turbofans tend toward higher bypass ratios to improve efficiency. For acoustic and fan efficiency reasons, it is desirable to reduce fan RPM or tip speed. However, a lower RPM increases low-pressure turbine loading, diameter and/or stage count. A fan directly driven by the low-pressure turbine limits the choice in fan speed because a slight reduction in fan speed for improved performance results in poorer performance in the low-pressure turbine.